The 2017 Nobel Prize in Physiology or Medicine was awarded to American biologists Jeffrey C. Hall, Michael Rosbash, and Michael W. Young for their discoveries of the molecular mechanisms that control circadian rhythm in organisms. Their work began in the 1980s with the study of fruit flies, from which they were able to identify the proteins that controlled the flies’ daily biological rhythms. Though flies and humans are vastly different, based on the Nobel laureates’ findings, it is now determined that the circadian rhythm of multi-cellular organisms function in similar ways. Circadian rhythm contains organisms’ “biological clocks” and is the self-sustaining system that provides organization to biological processes. It allows for animal, plant, and human life to prosper.
We’ve gathered these facts about circadian rhythm, its importance to daily life, and the impact that disrupted rhythms have on humans.
- Circadian comes from the Latin words meaning “about” (circa) and “one day” (diem), and the circadian rhythm maintains the cyclic variations that organisms undergo over twenty-four- to twenty-five-hour periods called tau. External cues called zeitgebers “entrain” the circadian system to the twenty-four hour solar day, and sunlight (and lack thereof) is the main zeitgeber for humans. The ebb and flow of hormones is synchronized for use during daylight hours.
- Animal research such as that of the Nobel laureates’ determined that the molecular machinery behind the circadian rhythm exists in each cell of the suprachiasmatic nucleus (SCN) in the hypothalamus, a small group of neurons in the brain. Studies in which SCN maintained in cell cultures still generated a twenty-four hour rhythm provide further evidence for this fact, though outside light is necessary for the resetting of the rhythm to keep the organism in sync with the solar day.
- Exposure to artificial lighting and light pollution during night hours – time meant to recuperate and rest before the daytime cycle starts again – can greatly affect the health of humans. This is especially impactful on the elderly, whose weaker circadian rhythms can become confused as a result of light pollution. In 2009, the American Medical Association officially identified light pollution as a health risk.
- Circadian rhythm is one of two processes that regulate the timing of sleep and wake. It interacts with the homeostatic sleep system to promote wake during the “biological day” and promote sleep during the “biological night.” The homeostatic drive to sleep, Process S, increases with time spent awake, while the circadian drive, Process C, increases or decreases depending on the time of day.
- Because circadian rhythm affects the sleep cycle, studying the circadian rhythm of individuals can determine if someone is a morning or a night person, especially in children. The assessment of a person’s circadian rhythm and sleep cycle is called a chronotype, which is conventionally broken down into three groups: morning people or “larks,” evening people or “owls,” and those who fall in between, “neither.”
- Sleep disorders such as the delayed sleep phase disorder (DSPD) and advanced sleep phase disorder (ASPD) are associated with circadian rhythms timed too late and too early. Bright light therapy and melatonin administration are two treatments for these disorders.
- Jet lag is another frequently-experienced circadian rhythm disorder. The abrupt transport from one time zone to another presents a new set of external time cues to the individual, causing a feeling of “shifted time” and can lead to insomnia, poor mood, and loss of appetite. Approximately two-thirds to three-quarters of travelers experience jet lag after trans-meridian travel.
- Featured image credit: “night and day” by qimono. CC0 via Pixabay.